Study On Low-Temperature Thermal Desorption Of Hydrophobic Organic Pollutants From Soil Using Gas Chromatographic Method

In recent years,low-temperature thermal desorption(150-315℃) technology has been widely used for the remediation of soil contaminated by volatile organic contaminants(VOCs) and semi-volatile organic pollutants(SVOCs).Although some researches have discussed the thermal desorption mechanism,some issues are still need to be clarified,such as,(1) experimental information about different soil/pollutants heat treatment systems,(2) solute transfer process and thermodynamic properties,and(3) the effects of operating conditions on thermal desorption rate and efficiency.In fact,thermal desorption of pollutant from the soil is the process that solute diffuses and translocates among the soil particles under heating and air-blowing conditions.The transfer process is similar to gas-solid chromatographic separation process.Based on this,we developed a new method simulating the transfer behavior of hydrophobic organic contaminants among soil particles under heating condition-Soil Column Gas Chromatography(SCGC) method.Then it was applied to studying the desorption kinetics,thermodynamics and molecular mechanism of hydrophobic organic pollutants among soil particles,and forecasting the soil organic carbon-normalized distribution coefficients(Koc).Firstly,the thermal desorption behaviors of 16 kinds of aromatic compounds from soil particles were measured.For each solute,capacity factor(k') and lag factor(T_f) were measured as the index of thermal desorption rate and thermal desorption efficiency, respectively.It was found that,the slow desorption process of organic pollutants from soil was conformed to the double constant equation,and logk' and T_f both linearly decreased with the increase of desorption temperature.Secondly,for better understanding the adsorption/desorption of organic pollutants in soil,linear salvation energy relationships(LSER) were used to quantitatively characterize the molecular interactions between organic pollutants and soils based on the correlations between solute molecular descriptors and retention factors measured by the SCGC.The results showed that,the solute size(logL¹⁶) and the HBA basicity(Σβ^H) were the two dominant factors governing the retention of organic pollutants on soil. Moreover,the molecular interactions involved in adsorption of organic pollutants on soil-organic compounds were characterized by LSER.Finally,a multi-column SCGC method was developed for prediction of Koc using the molecular descriptors obtained from retention parameters measured on different SCGC,and the results were compared with those from the traditional single-column SCGC.It was found that,the traditional single-column SCGC method was suitable for predicating the physicochemical parameters of congeneric chemicals,while the multi-column SCGC method could describe more molecules retention parameters,and thus could improve the prediction accuracy.